RXTE Discoveries

In the fiery machinery of the night sky, where neutron stars and black holes
wrapped in binary systems can flare and burst randomly from day to day,
astronomers have uncovered a predictable mathematical pattern in the X-ray
light emitted over the course of time.

Drs. Patricia Boyd and Alan Smale of NASA Goddard Space Flight Center in
Greenbelt, Md., have followed the history of X-ray emission from three binary
star systems over the last several years and uncovered a unifying concept:
The number of days between the low points of emission in each binary system
is random yet always based on multiples of a single constant.

The scientists say this never-before-seen pattern reflects the physics of how
matter swirls about and finally pours onto a neutron star or into a black
hole. They present their findings today at the 198th Meeting of the American
Astronomical Society in Pasadena, Calif.

"Neutron stars and black holes can be simultaneously predictable and random,
like a dice roll," said Boyd. "After many rolls, statistics tell us something
about the dice, that they each have six unique sides. Likewise, in binary star
systems, we see that lengths of the long variations (the dice rolls) can be
characterized over time by the dynamics of the two stars (the shape and
numbers on the dice)."

To obtain an uninterrupted history of a binary star system -- so as not to
miss a single dice roll -- the two scientists depended upon an instrument
aboard NASA's Rossi X-ray Timing Explorer called the All-Sky Monitor (ASM).
The ASM has assembled a continuous, five-year digital record of nearly all
local star systems known to flicker in X-ray radiation.

Black holes and neutron stars -- known as compact objects because they contain
great mass confined within a small region -- often reside in binary star
systems, sharing an orbit with a healthy, hydrogen-burning star. The compact
object is a strong source of gravity. Sometimes, when the orbits bring the two
companions close together or when the healthy star flares, gravity pulls gas
from the star toward the compact object. The journey, arduous enough for the
gas to glow hot in X-ray radiation, follows a path called an accretion disk.

Matter swirls around the compact object in the accretion disk like water going
down the drain. Because a black hole is invisible and a neutron star is so
tiny (only 10-20 kilometers across), astronomers best learn about these
objects from the dynamics of the very visible accretion disk.

X rays from accretion disks fluctuate. Some accretion disks exhibit periods
that reveal the geometry of the system. (The light disappears and reappears as
if it is moving behind something.) Sometimes accretion disks flare randomly,
which points to a disturbance somewhere in the system. (Gas builds up and
explodes.)

Boyd and Smale have uncovered a new tool to probe the physics of the accretion
disk, one that combines the predictability of geometry and the randomness of
disk disturbances. Their subjects are two probable black holes, Cygnus X-3 and
LMC X-3, and one neutron star, Cygnus X-2.

Cygnus X-2 has an orbital period of 9.8 days. Boyd and Smale found that the
time between minimum X-ray brightness is always a whole-number multiple of
9.8 -- for example 77.7 days, 58.8 days or 49 days, which are 8, 6 and 5
times 9.8. One cannot predict what multiple will come next; this is random.
The orbital period and the presence of whole-number multiples, though, are
constant.

Long term variations in LMC X-3 and Cygnus X-3 follow the same general rule:
the lengths of the variations are always a whole number multiplied by a
constant. Finding similar behavior in such different systems implies that the
mechanism for disk disturbances must be tied to something as predictable as a
clock.

What could cause such clockwork in a chaotic, flaring system? The clumpiness
and angle of the accretion disk may be one factor. Scientists believe that
accretion disks can be warped and tilted from the plane where the two stars
orbit. Gravity makes a tilted disk wobble like a spinning top. If a clump in
the accretion disk passed between the two stars as the disk wobbled, the
increased gravitational forces might set off the mechanism that disrupts the
accretion disk.

The theoretical details of weaving together both random and predictable
behavior have yet to be worked out.

"In binary systems, when a third body is sent into the mix, we see similar
statistics in the overall dynamics," said Boyd. "When the interloper gets
close to the two stars, there's a chance it may exchange enough energy to be
hurled away from the system. If not, it comes around again, and gets another
chance at either being thrown off, or staying bound. If an accretion disk
clump is dense enough to act like a third body, then something similar might
account for how the disk disruptions are timed in these systems."

"The interplay between periodic and random components in these systems is a
puzzle," said Smale. "We've only now been given the pieces, thanks to the ASM.
Future ASM data will either show the pattern to continue or reveal an
even more complex behavior."

Boyd and Smale work within Goddard's Laboratory for High Energy Astrophysics
through their appointments by the University of Maryland, Baltimore Country,
and the Universities Space Research Association, respectively. The Rossi
Explorer was launched in December 1995. Rossi's All-Sky Monitor was built by
the Massachusetts Institute of Technology.